With an electrically assisted bicycle for example, a motor is driven in accordance with a pedal pedaling force and an assist ratio that has been set beforehand. In a simple example, there are cases where assist torque, obtained by multiplying a pedal input torque corresponding to pedal pedaling force by an assist ratio according to pedal pedaling force. However, control is also performed to gradually reduce the assist ratio in accordance with vehicle speed when a predetermined vehicle speed or higher is reached, in order to prevent the assist from suddenly cutting out in cases where using the assist for high-speed riding is legally forbidden.
FIG. 1 illustrates change over time of assist torque in a case of simply multiplying pedal pedaling force by assist ratio. In FIG. 1, the dotted line represents the pedal input torque, and the solid line represents the assist torque obtained by multiplying this pedal input torque by the assist ratio corresponding to the pedal pedaling force. Thus, since the pedal input torque is input in an undulated form, the assist torque is also an undulated form if just multiplied by an assist ratio according to the pedaling force. In such a case, the assist is weak at points where pedal pedaling force cannot be applied, so the sensation of assistance is not good. Also, the assist torque is temporarily great, the peak of the motor driving current is also great, so this is driving with greater loss from the perspective of electric efficiency, as well.
Note that Japanese Patent No. 3955152 discloses a technology in which riding and driving can be performed with an optimal assist ratio constantly being decided, in speedy following response to change in riding environment and change in detected manpower driving force, whereby natural and smooth riding can be realized without giving the operator an unnatural sensation regarding the comfort of the ride of the vehicle. Specifically, a control device of a vehicle with an assisting driving device is a control device of a vehicle with an assisting driving device which provides a vehicle driving unit for performing driving with manpower driving force which pulsates in accordance with rotational operation of a pedal crank and assisting power from an assisting force driving unit, the control device including a detecting unit for detecting manpower driving force; a calculating unit for calculating the amount of work due to the manpower driving force during a period where the detected manpower driving force is at or above a threshold value for restricting assisting force; a correcting unit for obtaining assisting force by, after a point where the manpower driving force is at or below the threshold value, correcting the output current corresponding to the detected manpower driving force, so as to obtain assisting force; and a control unit for effecting control so as to output the corrected assisting force. With such control, there is a problem in that restriction is applied even at the point of starting to pedal, when great torque is desired.
Further, Japanese Unexamined Patent Application Publication 8-295285 describes technology for improving drivability when climbing hills and the like. Specifically, in a case of configuring a driving device for a power assisted vehicle including a manpower driving mechanism for driving a wheel by pedal pedaling force, and an assisting driving mechanism which provides assisting driving force corresponding to the magnitude of the pedaling force to the wheel, included are a pedaling cycle detecting unit for detecting a cycle tn (t1 through t5) of the pedal pedaling force, and a delay time control unit which makes a provided delay time of the assisting driving force (tn*constant C) to be proportionate to the pedaling force cycle tn. According to this technology, providing of the assisting driving force is delayed such that the time of the maximum assisting driving force approximates the minimum pedaling force, so a uniform and constant combined force can be supplied wherein the valleys in pedaling force have been supplemented by assisting driving force. Accordingly, fluctuations in the combined force is suppressed, the loss in speed due to values in combined force when climbing hills and so forth is reduced, and stable riding can be achieved. However, the assisting driving force is added in a delayed manner, and accordingly cannot handle starting from a stopped state. In the same way, cases other than cases where great assistance is necessary, such as when climbing hills and so forth, cannot be suitably handled.
Further, Japanese Patent No. 3301811 discloses technology for increasing the amount of electricity which the battery can discharge in a case of controlling motor output in accordance with pedaling force, so as to extend the battery life. Specifically, a bicycle with an electric motor, in which output of an electric driving system is controlled in accordance with change in human pedaling force, includes a pedaling force detecting unit for detecting pedaling force, a motor output calculating unit, configured of a CPU, for deciding constant electric motor output over a next cycle, based on measured values of pedaling force within one cycle obtained each predetermined time dt, and an output control unit configured to generate the obtained motor output. However, in the case of performing processing every predetermined time, suitable assistance which tracks fluctuating medal pedaling force cycles is not necessarily performable.